The present invention relates to a technique to image an LSI pattern onto an energy-sensitive material on a substrate and more specifically to an exposure system and method in which an exposure area is divided into subareas and the subareas are exposed in a step and repeat fashion using shaped beams.
With conventional various exposure techniques, an area to be exposed is generally larger than a unitary area that can be covered by exposure systems. For this reason, each area to be exposed is divided into subareas and each of the subareas is exposed in sequence. In the case that an area corresponding to a transferred pattern is constituted by a plurality of divided areas, it is very important how well the boundary portions of adjacent exposure areas are to be connected.
It has been known that in a system in which an exposure area is exposed by using light and combining unitary exposure areas, mask patterns are transferred so that adjacent exposure areas overlap each other in their boundary portions. According to this method, a problem of pattern failure is eliminated in which exposure dosage and position change in step-like manner in the overlapping zone of the exposure areas. To be specific, when two exposure areas are arranged to overlap each other in their boundary portions, the exposure dosages are adjusted so that the exposure in the overlapping zone of one exposure area is decreased progressively toward the end of that area and, in the other exposure area, the exposure is decreased in the opposite direction. As a result, at the termination of exposure, the sum of exposure dosages at any point in the overlapping zone of two exposure areas becomes constant. It therefore becomes possible to prevent pattern size and position from changing in step-like manner in the overlapping zone.
However, such a method as described above has not been able to be used with a step and repeat exposure system that controls exposing energy based on beam blanking time management.
In the step and repeat exposure system, each of shots, the minimum unit of exposure, is exposed in a different size but with a predetermined exposure dosage, by a predetermined or shaped beam. The system suffers from a limitation on the minimum beam irradiation time. The exposure dosages are treated as digital data (as opposed to analog data) and hence set in steps.
Having such features, the step and repeat (including step and scan) exposure system has no ability to, in exposing the overlapping zone, change progressively the exposure dosages from a constant value to zero as in photolithography. In electron-beam step and repeat lithography, therefore, the use of the overlap exposure as utilized in photolithographic techniques has been limited to the following ways.
One of conventionally proposed methods is to, in exposing each of overlapping adjacent exposure areas, expose shots of a pattern that resides across the boundary between the areas with half of exposing energy for the non-overlapping portions of the exposure areas, thereby improving the exposure accuracy in the boundary portion of the exposure areas. With this method, additional data processing is required to select patterns that reside across the boundary, making the overall data processing complex. In addition, there is a limit on the effectiveness of improvement in accuracy.
Another approach has been adopted which reduces and makes uniform the effects of the boundary portion by changing the boundary position and making multiple exposure. With this method, the degree to which exposure dosages change in steps in the boundary portion is reduced. However, as the multiplicity of exposure increases, the time required for exposure processing increases. In addition, the effect of step-like changes in exposure dosage still remains. Thus, a sufficient reduction of pattern failures cannot be effected.
Thus, there have been various proposals in the field of step and repeat lithography for improving the accuracy of connection between patterns in the boundary portion of exposure areas. However, these methods cannot be applied to the step and repeat lithography without modifications or cannot achieve sufficient effectiveness in the step and repeat lithography.